Pond without Fish

• Filters sizes are determined based on water flow rate measured in gallons per hour (gph). A filter should recirculate half of the water in a pond each hour. For a 1,200-gallon pond, the filter's flow rate must be at least 600 gph.
  
  

Pond with Fish

• If the pond has koi or other fish, the entire volume of the pond should recirculate one to two times each hour. A 1,200-gallon pond would require a filter with a 1,200 to 2,400 gph flow rate. If there is a large population of fish or if the fish are big, the filter should be even larger to ensure adequate filtration.
  
  

Considerations

• A pond with considerable sun exposure will likely harbor more algae than one in the shade. Increased algal populations necessitate a larger filter.